The present invention relates to plasma generation equipment, and more particularly to apparatus and methods for measuring radio frequency (RF) power supplied to an RF plasma generator.
To obtain better control of etching or deposition characteristics of a silicon wafer or other workpiece in a plasma chamber, it is conventional to employ a probe at a power input to the plasma chamber to detect voltage and current of a radio frequency (RF) wave as it enters the plasma chamber. One such probe utilizes a separate probe voltage and current pick-up device to sample the voltage and current of applied RF power as it enters an input of the plasma chamber. The probe pick-up has a shield or housing that electrically seals the pick-up. A voltage pick-up board inside the shield or housing is coupled via triax cables to a probe circuit arrangement housed in a separate chassis. A local oscillator and a pair of mixers produce separate baseband voltage and current signals, which are digitized utilizing a matched stereo analog to digital (A/D) converter and digitally processed utilizing a digital signal processor (DSP). The DSP is coupled via an interface to a feedback loop of the local oscillator to form a closed loop, and is also coupled to an external serial interface. The latter may in turn be coupled to controls for the plasma process arrangement.
Probes of the type described above provide satisfactory performance in many applications. However, group delays resulting from the signal processing performed in the probe circuit arrangement may reduce the speed or stability of some control systems.
There is therefore provided, in one aspect of the present invention, a radio frequency (RF) probe head apparatus for measuring voltage and current of an RF signal in a sampled transmission line. The probe head apparatus includes a conductive housing, a bus inside the conductive housing, a pair of connectors mounted on the conductive housing and configured to pass an RF signal into and out of the housing via the bus, a voltage pick-up board within the housing, and a current pickup board within the housing. The voltage pickup board has an analog processor responsive to an electric field around the bus to produce a first DC output indicative of a root-mean-square (RMS) value of the electric field. The current pick-up board has a second analog processor responsive to a magnetic field around the bus to produce a second DC output indicative of an RMS value of the magnetic field.
In another aspect, the present invention provides an RF probe head for measuring RMS electrical and magnetic fields in a sampled transmission line. In this aspect, the probe head includes a conductive housing, a pair of RF ports configured to couple an RF signal through the housing, circuitry within the housing configured to produce a first DC output proportional to an RMS value of an electric field produced by the RF signal inside the conductive housing and to produce a second DC output proportional to an RMS value of a magnetic field produced by the RF signal inside the conductive housing, and a pair of ports configured to couple the first DC output and the second DC output out of the conductive housing.
In yet another aspect, the present invention provides a method for measuring RF power in an RF probe head. The method includes passing RF power through an RF probe head having a conductive housing, sensing electric and magnetic fields produced by the RF power within the conductive housing to produce a first electrical signal representative of the electric field and a second electrical signal representative of the magnetic field within the housing, and processing the first electrical signal and the second electrical signal to produce a first DC output representative of an RMS value of the electrical field and a second DC output representative of an RMS value of the magnetic field. All of the processing takes place entirely within the conductive housing.
Configurations of the present invention provide highly accurate and repeatable measurements of RF line voltage and current, even at high RF power levels. Moreover, group delay resulting from signal processing performed in probe circuit arrangements is reduced relative to known conventional configurations.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.